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Monthly Archives: January 2018

The South American continent was crucial to inspire Charles Darwin´s Origin of Species. During his travel, Darwin recognized the importance of two factors while developing his famous theory on how nature works: biogeography and history. In Darwin’s words:

“When on board H.M.S. Beagle, as naturalist, I was much struck with certain facts in the distribution of the inhabitants in South America, and in the geological relations of the present to the past inhabitants of that continent”.

Professor Christian Ehrenberg was the first to analyze diatoms from South America, in the Tierra del Fuego. Darwin was fascinated by Ehrenberg´s microscopic observations of diatoms in diatom-rich geological deposits (diatomite). Due to the old age of such deposits and known diatom forms identified, Darwin thought that diatom distributions should also be old, and probably having wide geographical distributions due to their presence in that extreme region of Earth. Therefore, the very first studies of South American diatoms, back in the 19th century, appear to be strongly related to historical biogeography, which is nowadays a hot topic in microbial biogeography discussions (Williams, 2011).

Since then, however, South American diatom diversity has never been thoroughly investigated. Despite the difficulty in studying high-elevation waterbodies due to topographic and climatological extreme conditions, the Andes mountains became the focus of the first diatom studies. The first floristic descriptions were made in the 1920s by Hustedt in Chile (1927), followed by Frenguelli (1930-1940) in Argentina, and Manguin (1964) in Perú. However, despite the scientific interest of a variety of aquatic ecosystems in the Andes for studies on the diversity of diatoms and their use as ecological indicators (e.g. in paleoclimatology), later diatom studies are surprisingly scarce in this region.

Two major monographs were recently published by Rumrich et al. (2000) and Metzeltin & Lange-Bertalot (2007) in the Andes (Figure 1), and adjacent Amazon lowlands (Figure 2), respectively. According to these authors, the Amazon lowlands contain much more endemisms (diatom species unique to a region) than Andean waterbodies, the latter characterized by more numerous cosmopolitan species. The idea that Andean diatoms have a high proportion of species shared with temperate regions of Europe and North America is also supported by other studies (e.g., Servant-Vildary 1986; Alvial et al 2008).

Figure 2. Lake Chiriacu, Ecuadorean Andes. Photo by X. Benito.

Figure 3. Lake Mandicocha, Ecuadorean Amazonia. Photo by X. Benito.

Recently, some authors have challenged the view that Andean diatoms are cosmopolitan, arguing that a higher number of endemisms should exist than those currently known (Maidana et al. 2009; Morales et al. 2012). Rumrich et al. 2000 identified 888 diatom taxa from 350 samples randomly distributed from Venezuela to Argentina along the Andes, while Morales et al (2012) found 228 taxa in a single sample from an Andean Bolivian stream (with many taxa not identified at species level). A recent study by Benito et al. (2018), analyzing a diatom metadatabase from the tropical Andes and adjacent lowlands, found a total of 1,086 taxa in 163 aquatic samples, including streams and lakes. The number of new taxa described from the tropical Andes has increased, likely due to recent advances in taxonomy (e.g. molecular markers, scanning electron microscopy), and misidentifications of original diatom type material have been reviewed (Morales et al. 2014). However, a great part of the South American diatom flora may well be yet to be discovered.

An example of endemic diatom species from tropical Andean lakes is Cyclostephanos andinus (E.Theriot, H.G.Carney & P.J.Richerson) P.M.Tapia, E.C.Theriot, S.C.Fritz, F.Cruces & P.Rivera (Figure 3). C. andinus is abundant in phytoplankton communities of Lake Titicaca (Bolivia/Perú) and is also found in other high-elevation deep freshwater lakes of the central Andes. Due its sensitive and direct response to high water lake levels (>35 m) and dilute freshwaters, this bioindicator species has fueled research in paleoclimatology (Fritz et al. 2012), biodiversity (Tapia et al. 2004), and evolutionary processes (Spanbauer et al. 2018) in South America. For instance, population size variability of C. andinus through the last 400,000 years responded to regional environmental change via punctated changes driven by global-scale climate variability (e.g. el Niño-Southern Oscillation, ENSO) that influenced lake level of Lake Titicaca. The past climate of Andean Altiplano was also reconstructed using sedimentary record of C. andinus, indicating that wet conditions in Tropical South America, as reconstructed by overflowing conditions in Lake Titicaca due to periods of increased precipitation, coincided with cold periods in high-latitude regions (Baker et al. 2001).

There is a clear potential for new studies in South America to address research questions related to diatom taxonomy, ecology, and biogeography. Future works will benefit from harmonised taxonomic data sets spanning geographically distinct regions to understand diatom diversity patterns and drivers in mountain and lowland settings. Every new study dealing with geographic distributions of microorganisms in general, and diatoms in particular, relate to concepts of endemism and cosmopolitanism. Only when geographic distribution of species can be reliably determined, ecological value and thus prioritization of regions that contribute disproportionately to maintain regional diversity are possible (e.g. Williams, 2011). Microorganisms are often neglected in conservation studies, despite their value for ecosystem functioning and structure; diatoms can be more widely and better used to support conservation planning. For the case of the Andes, the endemics diatom taxa inhabiting hipersaline lakes in the Bolivian Altiplano (‘salares’; Blanco et al., 2013) could serve as example of microorganisms indicators of reservoir environments to be preserved from increased human activities in the region (e.g. tourism).

Williams, D.M. (2011) Historical biogeography, microbial endemism and the role of classification: everything is endemic. In Fontaneto, D. (ed) Biogeography of Microscopic Organisms. Is Everything Small Everywhere? Cambridge University Press, Cambridge.